The invention relates to high current analog switches, and more particularly to integrated circuit analog switches the node voltages of which rapidly settle to within a few microvolts of their quiescent values.
The state of the art for high speed integrated circuit analog switches is generally indicated in U.S. Pat. Nos. 4,042,836 (Compton et al.) and 4,228,367 (Brown). Junction field effect transistors (JFETs) are widely used in integrated circuit analog switches. When high currents must be switched, relatively large JFETs must be utilized as the analog switching elements. Such large JFETs have large gate capacitances which are difficult to charge and discharge with sufficient speed to satisfy the requirements of certain high speed switching applications. The above patents disclose circuits which result in fast turn-on and turn-off of high current analog JFET switches despite their high gate capacitances. However, if the circuits disclosed in the above references are connected to analog busses with large capacitances, e.g. 100 to 1,000 picofarads, such circuits require very long times, e.g., ten to thirty microseconds, for their output voltages to settle to within a few microvolts of their analog input voltages.
There is an unmet need for a high speed, high current integrated circuit analog switch that produces fast turn-on and fast turn-off of the JFET switching elements and also provides rapid equalization of the analog input and output voltages to within a few microvolts while driving large capacitive loads.
There also is a need to maintain conservation of charge between the analog input and analog output terminals of analog switches when used in certain applications. For example, in a so-called "flying capacitor" circuit in which a capacitor is first charged through an analog switch from an input source, then disconnected from the input source, and the stored charge then is redistributed through a second analog switch to a second capacitor, and the voltage on the second capacitor then is sampled by a suitable circuit, it is essential that there be no parasitic current leakage associated with the second analog switch if the sensed voltage is to accurately represent the sampled input voltage.